Mixed Anionic M(S/X)6 Polyhedra as Symmetry-Breaking Motifs to Enhance the Infrared Nonlinear Optical Response in Y–M–S–X (M = Ga, Ge; X = Cl, Br)
Rare-earth infrared nonlinear optical (IR NLO) compounds have garnered significant attention due to their abundant electronic structures. However, their NLO efficiency has been relatively modest, attributed to the localization of valence electron shells in rare-earth metals. Traditional design strategies for IR NLO materials have predominantly focused on MQ4 (M = metal, Q = S or Se) tetrahedra, with limited exploration of MQ6 octahedra. We introduce a novel approach to enhance NLO effects by incorporating distorted mixed anionic octahedral M(S/X)6 groups. To illustrate this strategy, we undertake structure modification and optimization of IR NLO performance using the Y–Ga–Ge–S system as an example. The synthesis of a new rare-earth chalcogenide, Y6Ga2GeS14 (1), serves as a foundation. Further halogen atom substitution yields Y6Ga4.2S13.6Cl3.4 (2), Y6Ga2.83Ge2S16.75Cl (3), and Y6Ga2.83Ge2S16.5Br1.5 (4). All compounds demonstrate remarkable NLO performance, including moderate NLO coefficients (0.17–0.70 × AgGaS2), large laser-induced damage thresholds (LIDTs) (3.78–6.19 × AgGaS2), and a broader transmission range (0.5–14.9 μm). The incremental enhancement of NLO efficiency in compounds 1–4 can be attributed to symmetry breaking induced by halogen atom substitution and increased distortion of the octahedral M(S/X)6 motifs in their structures, validating the effectiveness of our strategy. Notably, this strategy concurrently increases the band gap and LIDT, overcoming the NLO–LIDT incompatibility, being strikingly different from other reported approaches. This work provides new insights into optimizing the NLO performance of rare-earth IR NLO compounds through the modulation of octahedral motifs.
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